This invention relates to an optical recording medium having a phase change layer and a method for recording information in such medium.
Highlight is recently focused on optical recording media capable of recording information at a high density. Typical optical recording mediums include write once media which can be recorded only once and which can not be rewritten, and rewritable media wherein repeated rewriting has been enabled. Improvement in the recording density and increase in the data transmission rate are always required for an optical recording medium.
Write once media are unrewritable media which are adapted for use in documents wherein tampering of the information recorded therein may cause a serious problem as in the case of official documents. The most widely employed write once media are those using an organic dye for the recording material. Use of an organic dye, however, is associated with the difficulty of realizing a high transfer rate since recording sensitivity is likely to be insufficient when the recording is accomplished at a high speed by increasing the linear velocity of the medium. An organic dye also has relatively steep absorption spectrum and reflection spectrum, and a careful choice of the organic dye is required so that the organic dye chosen corresponds to the recording/reading wavelength. For example, when there is a high-end format which requires use of a recording/reading beam of shorter wavelength, a problem may arise that the medium of low-end format can not be recorded/read by the recording/reading beam adopted in the high-end format. There may also arise the problem that dyes corresponding to the recording/reading of shorter wavelength are difficult to design and purchase.
On the other hand, there are rewritable recording media of phase change type wherein the medium is recorded by changing the crystalline state of the recording layer by irradiating a laser beam, and read by detecting the change in the reflectivity induced in the recording layer by such change in the crystalline state.
In the phase change medium which can be rewritten by overwriting, amorphous record marks are formed by irradiating the medium with a laser beam of recording power level to melt the crystalline recording layer and quenching the molten recording layer to thereby form the amorphous record marks. In the erasure, the medium is irradiated with a laser beam of erasing power level to heat the recording layer to a temperature of not less than the crystallization temperature and less than the melting temperature followed by gradual cooling to thereby crystallize the amorphous record marks. Accordingly, the overwriting can be accomplished by irradiating a single laser beam with its intensity modulated. In the recording of such phase change medium at a high speed, the rate determining factor is crystallization speed of the recording layer, namely, the transformation speed from the amorphous to the crystalline phase. High speed recording can be realized by using a recording layer which crystallizes at a high speed while crystallization at an excessively high speed is likely to invite crystallization of the amorphous record marks, namely, destabilization of the record marks to adversely affect durability in the reading as well as storage stability. An excessively high crystallization speed may also invite the phenomenon of selferase wherein the record marks partly become recrystallized in the recording due to the heat conduction in in-plane direction as well as crosserase wherein record marks on the adjacent track are erased in the recording. As described above, it is difficult in a rewritable phase change medium to drastically increase the crystallization speed of the recording layer, and hence, remarkably increase the data transfer rate.
When a phase change medium is used as a write once medium, erasure (crystallization) is no longer required. When such medium is recorded at a high speed by increasing the linear velocity of the medium, increase in the crystallization speed of the recording layer in accordance with the increase in the linear velocity is unnecessary, and the problems as described above such as adverse effects on the storage reliability are alleviated. However, initialization of the recording layer will be difficult if the crystallization speed of the recording layer is excessively reduced for the purpose of increasing the storage reliability. The as-deposited phase change layer is generally amorphous, while the record marks formed by melting and quenching the crystalline recording layer are also amorphous. While both the as-deposited phase change layer and the record marks are amorphous, the stability of the amorphous phase is higher in the case of the as-deposited recording layer compared to the record marks. As a consequence, in the case of the overwriting of a phase change medium immediately after its production, crystallization of the area that has been irradiated with the laser beam of erasing power level is associated with difficulty. This is the reason why initialization (crystallization of the entire surface) is required before the overwriting. Difficulty in the initialization will invite increase of the production cost since a laser beam of high power is required and the medium can not be initialized at a high speed.
Also known in addition to those as described above are the write once media of the type wherein crystalline record marks are formed on the as-deposited recording layer, namely, on the as-deposited amorphous recording layer. In the medium of this type, the recorded medium has crystalline record marks formed in the amorphous recording layer, and the recorded state will be unstable if the stability of the amorphous phase is insufficient. However, when the stability of the amorphous phase is improved by reducing the crystallization speed, formation of the record marks will become difficult due to the difficulty of the crystallization. The medium of this type also suffers from the problem of difficulty in tracking servo in the recording since the amorphous recording layer of low reflectivity is irradiated with the recording laser beam.
Various proposals have been made to facilitate crystallization of the as-deposited amorphous recording layer or to speed up the erasure of the record marks. Proposals include provision of a layer in contact with the recording layer for promoting the crystallization of the recording layer, and constitution of the recording layer from a laminate of layers.
For example, Japanese Patent Application No. (JP-A) 92937/1989 discloses an optical recording medium comprising a recording layer containing Te or Se as its main component and a crystal nucleus-forming layer in contact with the recording layer, wherein apparent speed of nuclei formation near the melting point has been increased. There is also disclosed that the increase in the apparent nuclei formation speed of the recording layer enables erasure of the record marks at a higher speed. In claim 4 of JP-A 92937/1989, there is described that the crystal nucleus-forming layer is amorphous immediately after the production of the optical recording medium, and once crystallized by laser beam irradiation, the layer never becomes amorphous or immediately crystallized upon irradiation with the laser beam. In other words, the stable phase for this crystal nucleus-forming layer is the crystalline phase once the layer has been crystallized even if the layer went through repeated recording and erasing operations. JP-A 92937/1989 also describes that it is preferable that the crystalline phase of the crystal nucleus-forming layer after its crystallization is the same as the crystalline phase of the recording layer. Examples of JP-A 92937/1989 disclose combination of the recording layer comprising Te57In18Au25 and the crystal nucleus-forming layer comprising Te67Au33.
WO98/47142 discloses an optical information recording medium wherein a crystallization-promoting layer is provided in contact with the recording layer comprising a Gexe2x80x94Sbxe2x80x94Te-based alloy. This crystallization-promoting layer has a crystal structure of face centered cubic lattice which is the same as that of the recording layer, or a rhombohedral lattice which does not include Te. Initialization (crystallization) of the recording layer is not required in this medium since the recording layer is crystallized at the time of its formation owing to the provision of the crystallization-promoting layer and the recording layer in contact with each other. There is disclosed that the adjacent crystallization-promoting layer and recording layer turns out to be in mixed state. In Examples of WO98/47142, the recording layer comprises a composition based on Ge2Sb2Te5, and the crystallization-promoting layer contains PbTe, Bi2Te3, Sb, or Bi. In Comparative Examples, the crystallization-promoting layer contains W (body centered cubic lattice), Te (hexagonal system), Sb2TeSe2 (rhombohedral lattice), Sb2Te3 (rhombohedral lattice), Ag2Te (monoclinic system), or CrTe (hexagonal system).
JP-A 185289/1999 discloses a write once optical information-recording medium which has a phase change recording layer on at least one surface of the substrate, and a layer comprising a semiconductor material immediately on and/or under the recording layer. In this medium, when the recording layer is crystallized, the shape of the unit cell constituting the crystal face parallel to the substrate in the recording layer matches with the shape of the unit cell constituting the most dense face of the semiconductor material layer. The invention described in JP-A 185289/1999 attempts to reduce the jitter by providing such semiconductor material layer, and adequately selecting the material used for each layer so that absolute value of the lattice mismatch between the recording layer and the semiconductor material layer does not exceed 10%. JP-A 185289/1999 does not explicitly indicate the crystallization-promoting effect realized by providing the semiconductor material layer in contact with the recording layer. JP-A 185289/1999, however, describes that it has been estimated that, when the recording layer had been crystallized, deformation of the lattice that takes place at the boundary with the adjacent layer prevents crystallization, and hence, invites increase in the jitter. The compounds indicated in JP-A 185289/1999 as exemplary compounds for use in the semiconductor material layer include BaO, AgCl, BeTe, GaAs, AlAs, YSb, YP, ZnSe, ThS, SnAs, YSe, AgBr, ThP, LaS, ScSb, ThSe, CaSe, PbS, ScBi, ThAs, BiSe, InAs, YTe, GaSb, PbSe, SnSb, AlSb, CuI, SrSe, SnTe, ThSb, CaTe, BaS, LaTe, PbTe, BiTe, SrTe, AgI, InSb, CdTe, Sb2Te3, Bi2Se3, and Bi2Te3. The materials indicated for use in the recording layer include alloys containing at least one of Te, Sb and Se, among which Texe2x80x94Gexe2x80x94Sb alloys and Inxe2x80x94Sbxe2x80x94Texe2x80x94Ag alloys being indicated as the most preferable. The Inxe2x80x94Sbxe2x80x94Texe2x80x94Ag alloy used in the Examples is Ag2.6In3.7Sb64.2Te29.5. In the medium described in JP-A 185289/1999, crystalline pits (record marks) are formed in the amorphous recording layer. It should be noted that, unlike the WO98/47142, JP-A 185289/1999 does not explicitly refer to the state of the semiconductor material layer after completion of the medium. JP-A 185289/1999, however, discloses that it is not the interdiffusion between the recording layer and the compound semiconductor layer that takes place.
JP-A 226173/1998 discloses an optical recording medium which has a recording layer comprising a laminate of a Sb-based thin layer containing Sb as its main component and a reactive thin layer containing In, Ag and Te (and optional Sb) as its main components or Ge and Te (and optional Sb) as its main components, and wherein the mixing of both thin layers generates a phase change material. In this medium, the treatment of mixing both thin layers is generally conducted after forming the recording layer by continuously irradiating the layer with a laser beam. In the area where the layers have been mixed, the amorphous phase such Agxe2x80x94Sbxe2x80x94Te phase is dispersed in the Sb crystalline phase, and the reflectively is lower than that before the mixing but higher than the amorphous region (record marks). The medium is overwritten after the mixing treatment by the procedure normally used in a phase change medium. In the region which has been irradiated with the laser beam of erasing power level, crystallization into AgSbTe2 takes place to increase the reflectivity.
JP-A 73692/1999 discloses an optical recording medium which has a recording layer comprising a laminate of a Te-based thin layer containing Te as its main component and reactive thin layer containing Ge and/or Sb as its main component, and wherein the mixing of both thin layers generates a phase change material. In this medium, the treatment of mixing both thin layers is conducted after forming the recording layer by continuously irradiating the layer with a laser beam. In the area where the layers have been mixed, the amorphous phase such as Gexe2x80x94Sb phase is dispersed in the Te crystalline phase, and the reflectively is lower than that before the mixing but higher than the amorphous region (record marks). The medium is overwritten after the mixing treatment by the procedure normally used in a phase change medium. In the region which has been irradiated with the laser beam of erasing power level, crystallization into GeTe2 or Sb2Te3 takes place to increase the reflectivity.
JP-A 342629/1993 discloses an information recording medium wherein a high speed initialization has been enabled by providing an easily crystallizable auxiliary layer in contact with the recording layer comprising a phase change material. In this medium, the auxiliary layer has a composition containing at least 50 atom % of Se or at least 70 atom % of Te, and average composition of the auxiliary layer and the recording layer is Ge2Sb2Te5, GeSb2Te4, or In3SbTe2. In other words, the auxiliary layer and the recording layer of this medium reacts with each other to thereby constitute a typical composition of the phase change material.
JP-A 66668/1997 discloses a write once optical disk wherein a first Sbxe2x80x94Se-based thin film layer, a Bixe2x80x94Te-based thin film layer, and a second Sbxe2x80x94Se-based thin film layer are disposed one on another. This optical disk attempts to improve the recording sensitivity by using the composition which is somewhat different from the stoichiometrical composition for the first and second Sbxe2x80x94Se-based thin film layers. In this optical disk, reaction between the thin film layers is promoted by irradiating a laser beam to form a Bixe2x80x94Texe2x80x94Sbxe2x80x94Se-based quaternary alloy, and to thereby form record marks where the reflectivity has undergone a change.
In addition to the layers as described above, it is also known to use a non-metal layer for the purpose of promoting the crystallization of the amorphous phase change layer.
For example, JP-A 149322/2000 discloses a non-initialized phase change optical recording medium comprising a phase change layer and a crystallization-inducing layer provided in contact with the phase change layer. This crystallization-inducing layer is a crystalline light-transmitting layer. JP-A 149322/2000 discloses that overwriting of the as-deposited recording layer is enabled by the provision of the crystallization-inducing layer. JP-A 149322/2000 also indicates that, surface of a crystalline thin film generally functions as crystallization nuclei when the crystalline thin film is provided in contact with an amorphous thin film, and JP-A 149322/2000 makes use of such function. JP-A 149322/2000 indicates use of cerium oxide and zinc sulfide for the crystallization-inducing layer, and also, use of a ternary alloy comprising Ge, Sb and Te such as Ge2Sb2Te5 or a ternary alloy comprising In, Sb and Te for the recording layer.
SPIE Vol. 3401, 24-32 and JP-A 195747/1994 disclose that crystallization speed of the phase change layer can be increased by providing a layer of germanium nitride or silicon nitride in contact with the phase change layer having the composition near Ge2Sb2Te5.
An object of the present invention is to provide a write once medium having a phase change recording layer, which is stable in the recorded state and wherein crystallization of the recording layer is facilitated.
Such objects are attained by the present invention as described in (1) to (8), below.
(1) An optical recording medium having a recording layer comprising at least one phase change layer which can undergo amorphous-crystalline conversion and at least one functional layer in contact with the phase change layer, wherein the component constituting the phase change layer and the component constituting the functional layer undergoes a reaction to produce a reaction product when the phase change layer is heated to a temperature equal to or higher than the melting point of the phase change layer, wherein
the area where the reaction product has formed experiences a change in its optical reflectivity, and the optical reflectivity after the change is maintained even if the area wherein the reaction product has formed is heated to the crystallization temperature of the phase change layer.
(2) An optical recording medium according to the above (1) wherein said reaction product is not crystalline; and the area where the reaction product has formed experiences decrease in its optical reflectivity; and the state of decreased optical reflectivity is maintained even if the area where the reaction product has formed is heated to the crystallization temperature of the phase change layer.
(3) An optical recording medium according to the above (1) or (2) wherein the functional layer has an extinction coefficient at the recording/reading wavelength of at least 1.5.
(4) An optical recording medium according to any one of the above (1) to (3) wherein said functional layer is a crystalline layer.
(5) An optical recording medium according to any one of the above (1) to (4) wherein the relation:
Rcry greater than Rasd greater than Rrea
is satisfied when the reflectivity of the medium is Rcry in the crystalline area of the phase change layer; Rasd in the amorphous area of the phase change layer; and Rrea in the area where the reaction product has formed.
(6) An optical recording method for recording the optical recording medium of any one of the above (1) to (5) wherein
the recording layer including the phase change layer in amorphous state is irradiated with a laser beam of modulated intensity so that record marks comprising the reaction product are formed, and so that at the same time, the phase change layer becomes crystallized in the area other than the record marks.
(7) An optical recording method for recording the optical recording medium of any one of the above (1) to (5) wherein
the recording layer including the phase change layer in crystalline state is irradiated with a laser beam of modulated intensity so that record marks comprising the reaction product are formed, and so that at the same time, the phase change layer is maintained in its crystalline state in the area other than the record marks.
(8) An optical recording method for recording the optical recording medium of any one of the above (1) to (5) wherein
the recording layer including the phase change layer in amorphous state is irradiated with a laser beam of modulated intensity so that first record marks comprising the reaction product are formed, and at the same time, second record marks are formed by crystallization of the phase change layer, and the phase change layer is maintained in its amorphous state in the area other than the first and the second record marks.